Illumos 6288 - dmu_buf_will_dirty could be faster

Illumos 6288 - dmu_buf_will_dirty could be faster

6288 dmu_buf_will_dirty could be faster
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Paul Dagnelie <pcd@delphix.com>
Reviewed by: Justin Gibbs <gibbs@scsiguy.com>
Reviewed by: Richard Elling <Richard.Elling@RichardElling.com>
Approved by: Robert Mustacchi <rm@joyent.com>

References:
  https://www.illumos.org/issues/6288
  https://github.com/illumos/illumos-gate/commit/0f2e7d0

Porting notes:
- [module/zfs/dbuf.c]
  - Fix 'warning: ISO C90 forbids mixed declarations and code'
    by moving 'dbuf_dirty_record_t *dr' to start of code block.

Ported-by: kernelOfTruth kerneloftruth@gmail.com
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>